Durability is required for a toothed belt to be used in high-load transmission applications for rear wheel drive of motorcycles and the like.
Therefore, Patent Document 1 discloses a toothed belt which contains a tooth fabric using an ultrahigh molecular weight polyethylene fiber, a carbon fiber cord and a back side composed of a rubber composition having a JIS-A hardness of 87 degrees or more, prevents damages of the tooth fabric and tooth parts, and affords excellent durability in high-load transmission applications. Moreover, Patent Document 2 discloses a toothed belt which contains a tooth fabric using a polytetrafluoroethylene (PTFE) fiber, a carbon fiber cord and a back side composed of a rubber composition having a JIS-A hardness of 85 degree or more, and affords excellent durability in high-load transmission applications. In addition, Patent Document 3 discloses a toothed belt which contains a cord of a Lang-twisted carbon fiber having a particular twist factor and improves bending fatigue resistance.
However, although the toothed belts of Patent Documents 1 and 2 use a carbon fiber cord having excellent bendability, its twist structure is limited to single-twisted one or plied one, and bending rigidity thereof is high as compared to Lang-twisted one. Also, the JIS-A hardness of the rubber composition constituting the back side is set to such high hardness as 85 degrees or more or 87 degrees or more. Accordingly, the toothed belts of Patent Documents 1 and 2 have problems that flexibility of the entire belt is impaired, a sufficient bending fatigue resistance is not obtained, and severance of the belt is caused due to back side cracking.
On the other hand, although the toothed belt of Patent Document 3 achieves a certain flexural fatigue resistance, the cord diameter is as thin as about 1.0 mm, so that the use is limited to applications in which belt effective tension is up to about 3,500 N (e.g., OHC drive of automobiles, etc.). That is, when the toothed belt of Patent Document 3 is used under high-load conditions, failure with tooth crack occurs due to elongation of the belt since the cord diameter is small. Therefore, the toothed belt of Patent Document 3 cannot be used in high-load transmission applications, such as rear wheel drive of a motorcycle in which effective tension imparted to the belt is from 4,000 to 10,000 N. Furthermore, in dimension design of the belt, the tooth pitch of the belt and the PLD (Pitch Line Differential: distance from the bottom of the belt tooth to the center of the cord) are usually designed so as to coincide with the tooth pitch of a pulley, in consideration of the size of the pulley. In a large tooth form having such a tooth pitch as 14 mm, when the cord diameter is small as in the case of the toothed belt of Patent Document 3, the distance from the bottom of the belt tooth to the center of the cord becomes short and thus proper PLD that matches a pulley cannot be obtained. Then, there arises a problem that the tooth pitch of the belt does not coincide with the tooth pitch of the pulley. When the belt is used in such a state under a high-load condition, since there is a difference in the tooth pitches between the pulley and the belt, abnormal wear occurs on the pressure surface and tooth bottom surface of the belt and, as a result, there arise problems such as tooth crack and severance. Moreover, abnormal noise is generated due to large belt swing to the pulley diameter direction at the time of belt engagement. On the other hand, even in the case where the cord diameter is small, PLD can be increased by thickening the tooth fabric but the rubber content is decreased by an increase of the tooth fabric, so that tooth crack resistance is lowered.